Lifts are typically used to raise and lower heavy loads. Hydraulic lifts use hydraulic power units to control the pressure level of hydraulic fluid delivered to the lift and, accordingly, to raise or lower the lift. As used herein, hydraulic fluid means any fluid which can be used in a hydraulic system, including oil, emulsions, water, and synthetic fluids. Such power units typically have a motor attached to a pump which pulls the hydraulic fluid from a reservoir and delivers it to the lift. As hydraulic fluid is delivered to the lift, the fluid pressure increases until it overcomes the load on the lift, thereby raising the lift. To lower the lift, the motor is stopped and a return valve actuated which returns hydraulic fluid from the lift back into the reservoir.
Auto-hoist lifts typically have lifting members which engage the load to be raised and lowered and are controlled by a hydraulic power unit. The lifting members are attached to hydraulic cylinders which, in turn, are hydraulically connected to the power unit. The pressure of the hydraulic fluid operates the cylinders and therefore controls the elevation of the lifting members. The power unit has a pump which may pressurize the hydraulic fluid, thereby raising the lifting members. Alternatively, the fluid pressure may be relieved, thereby lowering the lifting members.
Unfortunately, conventional power units used to control hydraulic hoists are bulky and unduly load the motor. The motor of a conventional hydraulic power unit must be oversized to meet torque requirements during start-up. When first energized under a given load, a motor uses auxiliary windings to obtain a normal operating speed. As a result, the motor is less efficient and must be oversized to handle the given load during start-up. Once the normal operating speed is reached, the auxiliary windings are no longer used and motor efficiency increases. Accordingly, the motors of conventional hydraulic power units must be oversized to meet the torque requirement for start-up rather than the torque load experienced at normal operating speed.
Conventional hydraulic power units have further problems with noise. In a typical power unit, the motor and pump are located directly above and adjacent to the reservoir. The operation of the power unit results in considerable vibration of the pump and motor, which is communicated to the direct vicinity of the power unit in the form of noise. Since hydraulic lifts and their power units are commonly installed indoors, motor noise has been the source of substantial annoyance and dissatisfaction.
In addition, motors used in conventional hydraulic power units are exposed to the environment, and therefore must rely on air in the vicinity of the power unit to cool the motor. These motors typically do not incorporate fans to blow air through the motor and therefore the interior of the motor is susceptible to overheating.
Conventional hydraulic power units also use motors having mechanical means for switching off the auxiliary windings. The mechanical means typically employs a centrifugal switch which uses a spring to cut off the auxiliary windings. Spring displacement, however, is affected by the medium which surrounds the spring. For example, if the spring is submerged in hydraulic fluid, the loading and displacement of the spring while the motor is operating are different than when the spring is surrounded by air. Accordingly, the mechanical means used by conventional power units to control the auxiliary windings is often affected by the medium surrounding the motor.